We do not live in a perfect world. Though most of our industry has worked toward Agile adoption, over the past 20+ years, there are still a lot of clients and managers that are comfortable with Waterfall and either will not or cannot change.
This Dilbert sums this challenge up nicely:
Most of us know better, but things are not always within our control or influence. We need to understand and accept this. Let the client (or manager/boss) be right. Dale Carnegie preached that, "you cannot win an argument". This rule applies to how we work and the methods we use.
Agilish is not a methodology like Agile and Waterfall are. But Agilish is our"Get Out of Jail Free" card when we are boxed into doing Waterfall for one reason or another even and Agile is more appropriate for our team's work.
Work ahead wherever possible, deliver to UAT/Pre-PROD and let the Waterfall mechanisms take over to deliver to production. That is an Agilish strategy that can work.